CSS-6973 Create juju jaas snap

[kian99]

Description

This PR introduces a new snap in JIMM's repository called juju-jaas. This snap is intended to be a plugin into the Juju CLI that would extend it's functionality for the JAAS controller.

The mechanism by which it does this is Juju Plugins and Snapcraft's content-interface. The juju-jaas snap shares a binary of the same name to a directory within the Juju Snap that is readable by the CLI. Because the name of the binary is juju-* the binary can be invoked via juju jaas <command>. It is still TBD whether it is okay to namespace the JAAS commands to juju jaas or whether all the JAAS commands should be surfaced as top-level commands under the Juju CLI. Regardless, the specifics for how our commands are invoked can be easily changed after this by introducing symlinks/bash scripts into our Snaps $SNAP/bin directory of the form juju-<command> e.g. juju-add-service-account.

Note that juju-jaas snap is built on the bare base to be as minimal as possible and the binary within is compiled with CGO_ENABLED=0 to ensure a statically linked binary which will avoid any issues if the build-base of our snap were to be different to the build-base of the Juju snap.

Fixes CSS-6973

Engineering checklist

Check only items that apply

• Documentation updated
• Covered by unit tests
• Covered by integration tests

Test instructions

Introduced the appropriate plug into the Juju Snap (will make that PR next and update this). Then compiled both snaps and installed them with sudo snap install --dangerous --devmode <snap-file> and then ran sudo snap connect juju:jaas-plugin juju-jaas at which point the juju-jaas commands became available, see below:

$ juju jaas --help
Usage: jaas [flags] <command> ...

Flags:
--description  (= false)
    Show short description of plugin, if any
-h, --help  (= false)
    Show help on a command or other topic.

Details:
juju jaas enables users to use JAAS commands from within Juju.

JAAS enables enterprise functionality on top of Juju to provide
functionality like OIDC login, control over many controllers,
and fine-grained authorisation.

Subcommands:
    add              - Add service account
    list-credentials - List service account cloud credentials

$ juju jaas add 1235
{}
ERROR no such request - method JIMM(4).AddServiceAccount is not implemented (not implemented)

[alesstimec]

LGTM if the juju team is OK with using the jaas-plugin vs a more generic plugin slot content

[kian99]

I've asked in the other PR just now whether it should be generic or not.

[ale8k]

<3

[ale8k]

Why not core22?

[kian99]

The Juju snap is on core20 and my machine cannot (for some strange reason) build core22 snaps. But more the former reason, though we don't need to be on the same core anyway.

[babakks]

Since we've renamed the CLI to juju jaas, we should also update the docs for individual commands. We can do it here or in another PR. Up to you.

[babakks]

Why not just copy to $SNAP/bin?

[kian99]

I'm not sure $SNAP/bin is the right location. Our other snapcraft.yaml does go install which installs the file to $GOBIN but that does not let you rename the binary, so I instead do it like this.